In order to mount a wheel on the hub of a motor vehicle, bolted connections are conventionally used. In the procedures most commonly used at the present time, threaded bores are conventionally provided in the flange of the hub and through bores are formed in the wheel for the purpose of fixing the wheel to the radial flange of the hub; the wheel, a flange of the brake rotor and the radial flange of the hub are placed axially adjacently to each other, and a set of corresponding bolts is inserted into the aligned bores of these three elements, the bolts being screwed tightly into the threaded bores of the hub.
According to another conventional arrangement, threaded nuts are used. The wheel, the flange of the brake rotor and the radial flange of the hub are placed axially adjacently to each other, and the bores in these components are aligned. Four or five screws are inserted from the axially inner (or “inboard”) side of the flange of the hub. Each screw has a head and a shank having a terminal threaded portion and an axially knurled portion near the head. The screws are force-fitted with radial interference into the circular axial bores formed in the radial flange of the hub. When the screws have been fitted, the rotor and then the wheel are fitted onto the terminal parts of the shanks of the screws from the outside, and finally outer nuts are screwed on with a specified tightening torque. The knurling serves to fix the screws to the flange of the hub with respect to rotation, enabling the specified tightening torque to be applied correctly.
In the motor vehicle industry there is an increasing demand for the reduction of the weight of vehicle components, with the aim of reducing fuel consumption and exhaust emissions. In order to reduce the overall weight of the wheel, and particularly of the rotating mass, the flange of the hub can be partially made of a material which is lighter than the steel forming the central part or tubular core of the hub. Typically, the flange can be made of light metal alloys (such as aluminium, titanium or magnesium alloys), metal matrix composites, polymers, or fibre-reinforced polymers. The core of the hub is made of a high toughness metallic material such as bearing steel or low-carbon steel. The connection between the core of the hub and the lighter flange can be provided, alternatively, by means of a positive connection, or by overmoulding, for example by a semi-solid casting process.
It is known that the aforesaid light materials generally have lower mechanical strength than the steel, which is conventionally used.
The heads of the screws exert axial stresses directly on the areas surrounding the bores on the inboard face of the hub flange. This causes a concentration of high stresses on small surface areas. If axial knurling is provided on the screws, the connection of the screws to the hub flange is found to be insufficiently strong if the material of the flange is less hard than steel, or if the material of the flange has a higher coefficient of thermal expansion than the fastening screw. In these circumstances, the rotation of the screws with respect to the flange in which they are accommodated can be avoided by providing mutual non-circular form engagement between the screws and the bores of the flange. However, this requires special machining operations on the flange and screws in order to create a specific shape, which increases production costs.
To compensate for the lower mechanical strength of light materials, these materials are used in combination with tubular steel inserts, which are incorporated in the flange or fixed to it in some other way. These inserts, distributed at angular intervals in the flange, are used to create the threaded surfaces of the bores into which the screws are subsequently screwed, the threaded shanks of the screws protruding at least partially from the outboard side of the flange, that is to say the surface facing towards the axially outer (or “outboard”) side. Alternatively, the tubular inserts can be retained axially and rotationally by means of an outer thread, or a forced radial coupling, using one or more axial ribs or grooves on the outer cylindrical surface of each insert. Axial retention is also provided by a shoulder formed at the axially inner end of the insert, which is adapted to bear against the axially inner face of the hub flange.
The tubular inserts can be assembled onto the flange after the forming of the latter, or can be placed in the mould before the moulding operation, which forms the hub flange. In the latter case, the flow of plastic material in the moulding cavity is not sufficient to ensure the good structural quality of the portions of the flange located around the insert. In particular, an imperfect flow of plastic material causes incomplete filling of the areas downstream of the inserts, with respect to the direction of the flow of material in the mould.
If the inserts are positioned after casting, externally threaded inserts are normally used. This usually requires the forming of a pre-threaded bore in the flange, or the use of externally self-tapping inserts, which are locked in the flange during insertion. Self-tapping inserts and the operations for their insertion require considerable expenditure. The axial retention is developed locally at the insert/flange interface, and gives rise to concentrated high stresses. This also occurs in the case of force-fitted inserts which each have one or more axial ribs on their outer cylindrical surfaces, for the purpose of preventing rotation of the insert with respect to the flange.
Similar arrangements to those discussed above are used to connect the outer flanged ring of a hub-bearing unit to the suspension standard of a motor vehicle.